Piezoelectric transducer assembly and method for generating an umbrella shaped radiation pattern

ABSTRACT

A transducer assembly which generates a toroidal umbrella shaped radiation pattern concentric with a selected axis is formed by a piezoelectric element mounted in a cylindrical resonant cavity defined by a Helmholtz chamber. Circumferentially spaced apart and radially aligned circular apertures are formed in the chamber&#39;&#39;s cylindrical side wall and a reflecting plate is spaced apart from the apertures a selected distance. The plate extends perpendicularly to the chamber axis. The umbrella shaped radiation pattern is produced by generating a plurality of discrete spherical radiation patterns, combining these patterns to form an annular radiation pattern and partially reflecting the annular radiation pattern. The transducer assembly is mounted within a cavity having side walls essentially perpendicular to the plate for additional reflected reinforcement of the radiation in an umbrella pattern.

1 1 PIEZOELECTRIC TRANSDUCER ASSEMBLY AND METHODFOR GENERATING AN UMBRELLA SHAPED RADIATION .RATIERN c, A ,4 .1

Ralph W. Goble, Eldora, Colo.

Sontrix, lnc., Boulder, Colo.

Nov. 5, 1973 [75] Inventor:

Assignee:

Filed:

Appl. N0.:

References Cited UNITED STATES PATENTS 8/1973 Zeutschel 310/9.l X 9/1973 Takahashi et al...

[11] 3,873,866 1 Mar. 25, 1975 Primary Examiner-Mark O. Budd Attorney, Agent, or F irm-Edwards, Spangler,

' Wymore & Klaas [57] ABSTRACT A transducer assembly which generates a toroidal umbrella shaped radiation pattern concentric with a selected axis is formed by a piezoelectric element mounted in a cylindrical resonant cavity defined by a Helmholtz chamber. 'Circumferentially spaced apart and radially aligned circular apertures are formed in the chambers cylindrical side wall and a reflecting plate is spaced apart from the apertures a selected distance. The plate extends perpendicularly to the chamber axis. The umbrella shaped radiation pattern is produced by generating a plurality of discrete spherical radiation patterns, combining these patterns to form an annular radiation pattern and partially reflecting the annular radiation pattern. The transducer assembly is mounted within a cavity having side walls essentially perpendicular to the plate for additional reflected reinforcement of the radiation in an umbrella pattern.

29 Claims, 9 Drawing Figures FIG.8

FIG.9

PIEZOELECTRIC TRANSDUCER ASSEMBLY AND METHOD FOR GENERATING AN UMBRELLA SHAPED RADITION PATTERN BACKGROUND OF THE INVENTION The present invention relates to piezoelectric transducer assemblies and methods for generating radiation patterns.

Piezoelectricity is pressure electricity and piezoelectric behavior is the characteristic of materials to deform upon the application of electrical signals or conversely to develop electricity whenever deformed by the application of pressure. Materials exhibiting piezoelectric behavior are naturally occurring or may be man made.

Heretofore, planar piezoelectric elements which vibrate at a natural resonant frequency have been em ployed in transducer assemblies, particularly ultrasonic transducer assemblies. When vibrating at their natural resonant frequency, such planar piezoelectric elements flex about a node defined thereon; the portion of the element on one side of the node always vibrating in a direction opposite to the direction of vibration of the portion of the element on the other side of the node. A planar piezoelectric element having this type of vibration generates or senses compression and rarefaction waves essentially in a direction perpendicular to its planar surface.

In order to increase the acoustical output of transducer assemblies employing such planar piezoelectric elements, structural arrangements have been devised which operate to cause the compression and rarefaction waves generated on opposite sides of the element node and also on opposite sides of the plane of the piezoelectric element to combine through constructive interference. As a consequence, a generally spherical radiation pattern is typically generated by these transducers directed from one planar side of the piezoelectric element along an axis perpendicular to its plane. In such a spherical radiation pattern, the energy level is at a maximum along the axis, decreasing first gradually then rapidly with increasing angular offset from the axis. For example. at a point 45 off the axis, the energy level has decreased by approximately six decibels from the level on the axis. Thus, it is apparent that the spher ical radiation patterns generated by these prior art transducer assemblies are basically unidirectional along the subject axis. Conversely, it is noted that these transducers when used to detect acoustical waves are equally more sensitive to waves received along this axis.

Transducers characterized by having essentially unidirectional radiation patterns can be advantageously employed in ultrasonic detection systems by mounting them in adjustable mounts since their axes of radiation may be set, and/or repositioned as necessary to effectively monitor an area to be protected.

In many instances. however, the unidirectional radiation pattern of such transducers is a definite disadvantage. One example is in a room where it is necessary to mount the transducer assembly on or in a ceiling looking straight down. As apparent, the floor area protected by the transducer assembly due to its unidirectional characteristic is relatively small. Further, if the room has low ceilings. there is a tendency for the acoustic waves generated by the transducer to reverberate between the floor and ceiling, particularly if these sur- SUMMARY OF THE INVENTION It is, accordingly, an object of the present invention to provide an improved piezoelectric transducer assembly suitable for use in ultrasonic detection systems which obviate the disadvantages arising from the unidircctional radiation characteristics of the aforementioned prior art transducer assemblies.

It is further an object of the present invention to provide an improved piezoelectric transducer assembly characterized by being operable to generate a toroidal umbrella shaped pattern of acoustical radiation at a selected frequency, such as an ultrasonic frequency. and conversely having corresponding sensitivity to acoustical energy waves of the same frequency.

It is also an object of the present invention to provide a method for generating a toroidal umbrella shaped radiation pattern symmetrically about a selected axis.

It is still further an object of the present invention to generate an enhanced transducer output along a path at an angle to a reflecting surface to provide coverage beyond the toroidal umbrella radiation pattern.

In accomplishing these and other objects, a plurality of circumferentially spaced apart sound sources. each of which generates essentially a spherical radiation pattern, are provided by a piezoelectric element mounted in a resonant cavity, commonly called a Helmholtz chamber. The sound sources are defined by circumferentially spaced apart circular apertures in the cylindrical side wall of the Helmholtz chamber. The apertures are spaced apart and aligned in a manner that their spherical radiation patterns add for forming an annular radiation pattern concentric with the longitudinal axis of the Helmholtz chamber. Spaced apart from the apertures a selected distance is a reflecting plate. The plate extends perpendicularly to the longitudinal axis of the Helmholtz chamber and reflects the portion of the radiation of the annular pattern directed thereon in such a manner that reflected radiation adds with nonreflected radiation to form from the annular pattern a toroidal umbrella shaped radiation pattern concentric with the axis of the Helmholtz chamber. In a preferred embodiment, a reflecting wall is provided spaced from the Helmholtz chamber apertures a predetermined distance and reflects the portion of the radiation of the annular pattern directed thereon in such a manner that the reflected radiation adds with the nonreflected radiation and the radiation reflected from the plate to enhance the formation of a toroidal umbrella pattern having an even higher output.

Additional objects reside in the specific construction of the exemplary piezoelectric transducer assembly hereinafter described and its method of operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation view partially cut away of a piezoelectric transducer assembly according to the present invention; 7

FIG. 2 is a plan view of the assembly of FIG. 1;

FIG. 3 is a side view of the piezoelectric element incorporated in the transducer of the assembly of FIG. I;

FIG. 4 is a plan view of the element of FIG. 3;

FIG. 5 is a side elevation view of the transducer of FIG. 1 illustrating a cross sectional view of the radiation pattern generated thereby within the resonant cavity of the assembly of FIG. 1;

FIG. 6 is a cross sectional elevation view of one of the circular apertures in the side wall of the resonant cavity of the assembly of FIG. 1 illustrating a cross sectional view of the radiation pattern emitted therefrom;

FIG. 7 is a side elevation view of the assembly of FIG. 1 shown mounted on a ceiling illustrating a cross sectional view of the umbrella shaped radiation pattern generated thereby;

FIG. 8 is a side elevation view partially in cross section of the assembly of FIG. 1 recessed in a cavity; and,

FIG. 9 is a view as in FIG. 8 wherein the cavity in which the assembly is recessed is defined by structure having beveled outer edges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings in more detail, there is shown in FIGS. 1 and 2 a transducer assembly generally identified by the numeral 10. The assembly 10 is made up of a resonant chamber 11, a transducer 12 and a re fleeting plate 13.

The resonant chamber 11 has both ends closed to define a resonant chamber and is of conventional con struction, being of the type generally referred to as a Helmholtz chamber. As shown in FIG. I, the upper positioned end of the chamber 11 is closed by the support 1211 supporting transducer 12 while its lower end is closed by end wall 14. The reflecting plate 13 extends to each side of the chamber 11 in a plane substantially perpendicular to the longitudinal axis of the chamber 11.

Mounted within the chamber 11 to extend symmetrically and perpendicularly across its longitudinal axis is the transducer 12. The transducer 12 is mounted upon support 12a in a conventional manner and includes a piezoelectric element 20, shown in FIGS. 3 and 4.

The element is a flat plate-like bender type of piezoelectric element, such as the bender type of piezoeelectric element made by Clevite Corporation under the name BIMORPH, which in response to an electrical field applied perpendicularly to its flat surfaces, flexes or bends as shown about its node 21. The element 20 is shown in FIG. 4 as being rectangular with a circular node 21. The location of the node 21 is shown in dashed lines in FIG. 4. As shown by FIG. 3, the element edge portions are always moving in the direction opposite to the direction of movement of the element center portion within the node 21. In FIG. 3, the element 20 is shown in its upwardly bent position in solid lines and in its downwardly bent position in dashed lines.

The piezoelectric element 20 has a selected, preferably ultrasonic, natural resonant frequency and is mounted in the transducer 12 for free vibration about its node 21. Included in the transducer 12 is structure which causes the compression and rarefaction waves generated on opposite sides of the element node 21 to be phase shifted so as to combine through constructive interference and reinforce each other. Additionally, the element 21 is held in the transducer 12 appropriately spaced from the adjacent surface of support 12a so that sound waves generated on opposite sides of the plane of the element 20 are reflected to constructively interfere and hence reinforce each other. Further, the transducer 12 includes electrical contacts and terminals 15a and 15b through which electrical signals may be picked off or applied to the opposite faces of the piezoelectric element 20.

It is noted that one suitable manner in which the piezoelectric element 20 may be mounted and held in the transducer 12 is disclosed in US. Pat. No. 3,704,385 issued Nov. 28, 1972, to Schweitzer et al.

When the piezoelectric element 20 is electrically excited at its natural resonant frequency, the transducer 21 operates in a conventional manner within the cham ber 11 to generate a spherical radiation pattern 30. The resonant frequency of the element 20 is here assumed to be an ultrasonic resonant frequency.

A cross sectional view of the spherical radiation pattern 30 is shown in FIG. 5. Sound vectors 31, 32, and 33 are there identified. The vector 32 lies on the longitudinal axis of the chamber 11 while the sound energy vectors 31 and 33 are offset 45 from the vector 32 on diametrically opposite sides of the spherical pattern 30.

The Helmholtz chamber 11 defines a resonant cavity of appropriate length and is axially adjustable with respect to support 12a to provide a resonant frequency corresponding to the resonant frequency of the piezoelectric element 20. As a consequence, the acoustical output of the transducer 12 is amplified by the resonant action of the Helmholtz chamber 11. The chamber 11 is retained in position with respect to support 12a by a clamping ring 18.

Formed a predetermined distance from the reflecting plate 13 are a plurality of cireumferentially spaced apart radially aligned apertures 16 in the cylindrical side wall of the Helmholtz chamber 11. Each aperture or opening 16 is a circle having a diameter equal to approximately one-half wavelength ofthe ultrasonic resonant frequency of the element 20. The wavelength of the resonant frequency is hereinafter referred to as A.

The apertures 16 are circumferentially spaced apart around the chamber cylindrical side wall with their centers 17 lying in a plane parallel to the plate 13 and a distance of approximately one It between their adjacent centers 17. For reasons hereinafter explained, the distance X between each aperture centerpoint l7 and the plane of the plate 13 measured along a normal to the plate 13 is equal to ma sine 0, where m is equal to any whole number, e.g. one, two, three, etc. and 6 is defined as the angle between the principal vector 36 and the vector, i.e. 38, of the energy to be reinforced.

In operation, the Helmholtz chamber 11 converts the spherical radiation pattern 30 generated along its axis by the transducer 12 into a plurality of substantially spherical radiation patterns 35 which are outputted by the apertures 16. Thus, each aperture 16 operates, as shown in FIG. 6, as a discrete sound source to generate a spherical radiation pattern having a maximum energy component along the axis indicated by the vector 36. The vectors 36 from the apertures 16 extend from the aperture centerpoints 17 parallel to the reflecting plate 13. For purposes of discussion, sound components or vectors 37 and 38 which define a plane perpendicular to the reflecting plate 13 are identified. Vectors 37 and 38 are positioned on diametrically opposite sides of the spherical pattern 35 at an angle of 6 relative to the aperture axis defined by the vector 36. It will be appreciated that maximum reinforcement will occur where 0 45; however, reinforcement can occur where 6 is other than 45 by appropriate spacing of the center 17 of apertures 16 from the surface of reflecting plate 13 to satisfy the relation of the distance X being equal to mk sine 0, where 0 is the angle between the principal vector 36 parallel to the plate 13 and the vector, i.e., 38, desired to be reinforced.

Due to the one A spacing between the aperture centers 17, the spherical patterns 35 of ultrasonic radiation add together in phase to form an outwardly radiating annular radiation pattern symmetrically around the longitudinal axis of the chamber 11. As this annular radiation pattern radiates outwardly, the portion of it containing the vector 37 soon contacts the reflecting plate 13, as shown in FIG. 1. The vector 37, as shown, strikes the plate 13 at an angle of incidence of 45. Ac cordingly, the distance an acoustical wave traveling along the vector 37 has traversed at the time of striking the plate 13 is given by the formula Z equals X divided by sine 45, where Z equals the distance traveled along the vector 37 and X equals the distance from the aperture centerpoint 17 measured along a normal to the plate 13.

As earlier mentioned, the distance X is selected to equal mk sine 0. Thus, by using the above formula, it is apparent that the distance Z equals mk sine 6 divided by sine 6. Accordingly, Z equals ma. Due to this fact, the acoustical waves reflected from the plate 13 along the path of the vector 37 add in phase with the acoustical waves traveling along vector 38 to form an umbrella shaped radiation pattern 40 around the axis of the Helmholtz chamber 11.

A cross sectional view of the umbrella shaped radiation pattern 40 is shown in FIG. 7. The transducer assembly is there shown with its reflecting plate 13 mounted on a ceiling 14 of a room. The axis 42 of the assembly 10 extends vertically downwardly from the ceiling 4l. The umbrella shaped toroidal radiation pattern 40 is generated symmetrically 360 around the axis 42 with its maximum energy components being along the vectors 43 which extend at 45 to the axis 42. As illustrated in FIG. 7, the radiation pattern 40 thus provides an umbrella of protection over a relatively large surface area of the floor 44. Also as indicated in FIG. 7, the radiation along vector 43 will be reflected off of surface 44 as 430, so that an object located at position A, normally beyond the immediate coverage of the um brella will be detected thereby. Also vector 43a will reflect off of ceiling 41 and thus may be repeated several times to greatly enlarge the area covered. This is even more pronounced witht multiple transducer assemblies.

It is noted that the transducer assembly 10 can also be operated in pickup mode. Referring, for example, to FIG. 7, where 6 is equal to 45, the maximum sensitivity of the assembly 10 in the pickup mode would be at 45 along the vectors 43 and the receiving sensitivity pattern would be essentially identical to the transmitting pattern there shown.

Referring to FIG. 8, the Helmholtz chamber 11 of the assembly 10 is there shown mounted in a cylindrical cavity 50. The cavity 50 may be defined in a room ceiling or be defined by separate structure for mounting on a wall or ceiling. The cavity 50 is preferably cylindrical in shape, being defined by a top wall 51 and side cylindrical wall 52. A top wall 51, if made of suitable material can serve as the reflecting plate 13, just as a suitable ceiling surface itself could serve as the reflecting plate. The side wall 52 is parallel to the axis of the chamber 11 so that acoustical waves traveling along the vector 36 are not reflected outside of the cavity 50. Further, it is apparent that the cavity must be large enough to permit the acoustical waves traveling along the vector 37 to be reflected there-out-of. The cavity is preferably of a size such that the acoustical waves traveling from the centerpoint of apertures 16 along vector 36 will be reflected such that the reflected waves are in phase with the transmitted waves and serve to reinforce same. Thus, the spacing from the apertures 16 and chamber 11 to the wall 52 of the cavity should be equivalent to "A, where n is equal to or greater than one wavelength A of the acoustical waves transmitted depending on the umbrella pattern desired. Thus, the portion ofthe radiation from apertures 16 which strikes wall 52 will be reflected such that the reflected waves are in phase with the transmitted waves and serve to reinforce same. While the wall 52 is preferably normal to the principal transmitted radiation vector 36, other angular relations may be desirable where the greatest sensitivity is desired along a vector where 0 is other than 45. The wall 52 is preferably positioned at an angle with respect to the plane of the aperture centerpoints which is equal to or greater than 90. The advantage of mounting the transducer assembly in a recessed location is to make it less vulnerable to being damaged. In FIG. 8, the chamber end 14 lies in the plane of the ceiling 42.

In FIG. 9, shield structure is shown defining the recess 50. The shield structure 60 is suitable for mounting on a ceiling and has beveled edge portions 61 to improve its aesthetic appearance and as pointed out above to protect same from being struck by foreign objects.

Thus, an improved piezoelectric transducer assembly has been provided which generates a toroidal umbrella shaped radiation pattern by the method of generating one spherical radiation pattern, producing a plurality of substantially spherical radiation patterns from the one, combining the plurality of patterns to form an annular radiation pattern, and reflecting portions of the annular pattern to form the toroidal umbrella shaped radiation pattern.

Although the piezoelectric transducer assembly herein shown and described is what is conceived to be the most practical and preferred embodiment of my invention, it is recognized that various modifications can be made therein in making a transducer assembly in accordance with the spirit of the invention which operates in an equivalent manner to obtain an equivalent result.

What is claimed is:

1. A piezoelectric transducer assembly for generating a radiation pattern of acoustical energy having an umbrella shaped cross section relative to a selected axis, said assembly comprising:

a resonant chamber having side walls and closed ends and a selected resonant frequency, said chamber being positioned with its longitudinal axis parallel with the side walls and defining the selected axis of said assembly, said chamber having a plurality of spaced apart sound transmitting apertures defined in its side walls about its longitudinal axis, the centerpoints of said apertures defining a plane substantially perpendicular to said selected axis;

piezoelectric transducer means mounted within said chamber for generating therein along said selected axis a spherial radiation pattern of acoustical energy at the resonant frequency of said chamber whereby similar radiation patterns of said acoustical radiation are emitted from each of said apertures along their center axes; and,

means defining a plane reflecting surface positioned to receive and reflect acoustical waves radiating from the centerpoints of said apertures towards said reflecting surface along a path of travel at a predetermined angle 6 to the plane of the centerpoints of said apertures, said plane reflecting sur' face extending perpendicularly to said selected axis of said assembly and being spaced apart from the plane of the centerpoints of said apertures a predetermined distance X at which acoustical waves at said resonant frequency radiating from said apertures at said predetermined angle to the plane of their centerpoints towards said reflecting surface are reflected in phase with mirror image acoustical waves which radiate from said apertures away from said reflecting surface, where 0 is the angle between the plane of the aperture centerpoints and the vector of the mirror image waves to be reinforced.

2. The invention of claim 1 wherein the predetermined distance X is defined by the equation X ma sine 6, where is the resonant frequency and m is a whole number.

3. The invention defined in claim 1 wherein the angle 6 is 45.

4. The invention defined in claim 1, wherein said apertures are circular.

5. The invention defined in claim 1, wherein said apertures are equidistantly spaced apart.

6. The invention defined in claim 1, wherein said apertures are circular and are equidistantly spaced apart at a distance between their centerpoints equal to approximately one wavelength at said resonant frequency.

7. The invention defined in claim 6, wherein the diameter of each of said apertures is equal to approximately one-half wavelength M2 at said resonant frequency.

8. The invention defined in claim 7, wherein said plane reflecting surface is spaced apart from the plane of the centerpoints of said apertures a distance measured along a normal to said reflecting surface equal approximately to a whole number of wavelengths at said resonant frequency multiplied by the sine of angle 6.

9. The invention defined in claim 8, wherein said resonant frequency is an ultrasonic resonant frequency.

10. The invention defined in claim 9, wherein said transducer means includes a flat plate-like bender type piezoelectric element.

11. The invention defined in claim 1, wherein said apertures are sized, shaped and spaced apart to emit substantially spherical patterns of radiation of acoustical energy at said resonant frequency which combine in phase with each other.

12. The invention defined in claim 11, wherein said plane reflecting surface is spaced apart from the plane of the centerpoints of said apertures a distance measured along a normal to said reflecting surface equal approximately to a whole number of wavelengths at said resonant frequency multiplied by the sine of 45.

13. The invention defined in claim 1, wherein said plane reflecting surface is spaced apart from the plane of the centerpoints of said apertures a distance measured along a normal to said reflecting surface equal to approximately a whole number of wavelengths at said resonant frequency multiplied by the sine of 45.

14. The invention defined in claim 1, including shield structure defining a cavity in which said assembly is recessed. said shield structure defining cavity side walls spaced from said chamber a predetermined distance with the side walls being at an angle with the plane of the aperture centerpoints equal to or greater than 15. The invention defined in claim 8, including shield structure defining a cavity in which said assembly is recessed, said shield structure defining cavity side walls spaced from said chamber a predetermined distance with the side walls being at an angle with the plane of the aperture centerpoints equal to or greater than 90.

16. The invention defined in claim 14, wherein the cavity side walls are spaced from the apertures in the side walls of the chamber a distance equivalent to ma, where m is a whole number.

17. The invention defined in claim 1, wherein the resonant chamber is cylindrical.

18. The invention defined in claim 16 where the sides of the cavity are parallel with said selected axis of said chamber.

19. The invention defined in claim 1 including a reflecting wall surrounding said chamber in concentric fashion and spaced outwardly from the apertures a distance equivalent to nlt, where n is equal to or greater than one.

20. The method of generating an umbrella shaped toroidal radiation pattern of acoustical energy of a selected wavelength around a selected axis, comprising:

generating in a selected plane perpendicular to said selected axis an annular outwardly radiating pattern of acoustical energy at said selected wavelength; and,

reflecting the acoustical waves of said annular radiation pattern in phase with the acoustical waves of said annular radiation pattern radiating to the other side of the plane of said annular radiation pattern whereby to produce said umbrella shaped toroidal radiation pattern.

21. The assembly of claim 19 including shield structure defining a cavity in which said assembly is recessed, said shield structure defining cavity side walls spaced from the source of said pattern a distance equal to ink, where m is a whole number.

22. The method of claim 20 wherein the acoustical waves of said annular radiation pattern radiating to one side of the selected plane of said annular radiation pattern are reflected from a plane reflecting surface in phase with the directly radiated waves radiating to the other side of the plane of said annular radiation pattern to reinforce same.

23. The invention defined in claim 20, wherein said annular shaped radiation pattern is generated by combining a plurality of similar spherical radiation patterns of acoustical energy at said selected wavelength.

24. The invention defined in claim 23, wherein the acoustical waves of said annular radiation pattern radiating to one side of the selected plane of said annular radiation pattern are reflected in phase with the acoustical waves of said annular radiation pattern radiating to the other side of the plane of said annular radiation pattern by a plane reflecting surface positioned parallel with the plane of said annular radiation pattern and psitioned a distance therefrom measured along a normal to said reflecting surface equal approximately to a whole number of wavelengths at said selected wavelength multiplied by the sine of 6, where 6 is defined as the angle between the plane of said reflecting surface and the vector of the outwardly radiating energy to be reinforced.

25. The invention defined in claim 24, wherein the angle 0 is 45.

26. The invention defined in claim 22, wherein the plane of said annular radiation pattern and said plane reflecting surface are spaced apart a distance measured along a normal to said reflecting surface equal approximately to a whole number of wavelengths at said selected wavelength multiplied by the sine of 45.

27. The assembly of claim 21 wherein the cavity side walls are parallel to the selected axis.

28. The method of claim 22 including reinforcing the directly radiating waves of said annular radiation pattern radiating to the other side of the plane of said annular radiation pattern by also reflecting acoustical waves of said annular radiation pattern by a plane reflecting surface positioned parallel to the selected axis.

29. A piezoelectric transducer assembly for generating an umbrella shaped toroidal radiation pattern of acoustical energy around a selected axis, comprising:

means for generating an annular shaped outwardly radiating pattern of acoustical energy at a selected wavelength. said generating means including a piezoelectric transducer element and being positioned to generate said annular shaped radiation pattern concentric with said selected axis, and. means defining a plane reflecting surface perpendicular to said selected axis positioned with respect to said generating means for reflecting a selected portion of said annular shaped radiation pattern to form therefrom said umbrella shaped toroidal radiation pattern around said selected axis.

i l l t 

1. A piezoelectric transducer assembly for generating a radiation pattern of acoustical energy having an umbrella shaped cross section relative to a selected axis, said assembly comprising: a resonant chamber having side walls and closed ends and a selected resonant frequency, said chamber being positioned with its longitudinal axis parallel with the side walls and defining the selected axis of said assembly, said chamber having a plurality of spaced apart sound transmitting apertures defined in its side walls about its longitudinal axis, the centerpoints of said apertures defining a plane substantially perpendicular to said selected axis; piezoelectric transducer means mounted within said chamber for generating therein along said selected axis a spherial radiation pattern of acoustical energy at the resonant frequency of said chamber whereby similar radiation patterns of said acoustical radiation are emitted from each of said apertures along their center axes; and, means defining a plane reflecting surface positioned to receive and reflect acoustical waves radiating from the centerpoints of said apertures towards said reflecting surface along a path of travel at a predetermined angle theta to the plane of the centerpoints of said apertures, said plane reflecting surface extending perpendicularly to said selected axis of said assembly and being spaced apart from the plane of the centerpoints of said apertures a predetermined distance X at which acoustical waves at said resonant frequency radiating from said apertures at said predetermined angle theta to the plane of their centerpoints towards said reflecting surface are reflected in phase with mirror image acoustical waves which radiate from said apertures away from said reflecting surface, where theta is the angle between the plane of the aperture centerpoints and the vector of the mirror image waves to be reinforced.
 2. The invention of claim 1 wherein the predetermined distance X is defined by the equation X m lambda sine theta , where lambda is the resonant frequency and m is a whole number.
 3. The invention defined in claim 1 wherein the angle theta is 45*.
 4. The invention defined in claim 1, wherein said apertures are circular.
 5. The invention defined in claim 1, wherein said apertures are equidistantly spaced apart.
 6. The invention defined in claim 1, wherein said apertures are circular and are equidistantly spaced apart at a distance between their centerpoints equal to approximately one wavelength lambda at said resonant frequency.
 7. The invention defined in claim 6, wherein the diameter of each of said apertures is equal to approximately one-half wavelength lambda /2 at said resonant frequency.
 8. The invention defined in claim 7, wherein said plane reflecting surface is spaced apart from the plane of the centerpoints of said apertures a distance measured along a normal to said reflecting surface equal approximately to a whole number of wavelengths at said resonant frequency multiplied by the sine of angle theta .
 9. The invention defined in claim 8, wherein said resonant frequency is an ultrasonic resonant frequency.
 10. The invention defined in claim 9, wherein said transducer means includes a flat plate-like bender type piezoelectric element.
 11. The invention defined in claim 1, wherein said apertures are sized, shaped and spaced apart to emit substantially spherical patterns of radiation of acoustical energy at said resonant frequency which combine in phase with each other.
 12. The invention defined in claim 11, wherein said plane reflecting surface is spaced apart from the plane of the centerpoints of said apertures a distance measured along a normal to said reflecting surface equal approximately to a whole number of wavelengths at said resonant frequency multiplied by the sine of 45*.
 13. The invention defined in claim 1, wherein said plane reflecting surface is spaced apart from the plane of the centerpoints of said apertures a distance measured along a normal to said reflecting surface equal to approximately a whole number of wavelengths at said resonant frequency multiplied by the sine of 45*.
 14. The invention defined in claim 1, including shield structure defining a cavity in which said assembly is recessed, said shield structure defining cavity side walls spaced from said chamber a predetermined distance with the side walls being at an angle with the plane of the aperture centerpoints equal to or greater than 90*.
 15. The invention defined in claim 8, including shield structure defining a cavity in which said assembly is recessed, said shield structure defining cavity side walls spaced from said chamber a predetermined distance with the side walls being at an angle with the plane of the aperture centerpoints equal to or greater than 90*.
 16. The invention defined in claim 14, wherein the cavity side walls are spaced from the apertures in the side walls of the chamber a distance equivalent to m lambda , where m is a whole number.
 17. The invention defined in claim 1, wherein the resonant chamber is cylindrical.
 18. The invention defined in claim 16 where the sides of the cavity are parallel with said selected axis of said chamber.
 19. The invention defined in claim 1 including a reflecting wall surrounding said chamber in concentric fashion and spaced outwardly from the apertures a distance equivalent to n lambda , where n is equal to or greater than one.
 20. The method of generating an umbrella shaped toroidal radiation pattern of acoustical energy of a selected wavelength around a selected axis, comprising: generating in a selected plane perpendicular to said selected axis an annular outwardly radiating pattern of acoustical energy at said selected wavelength; and, reflecting the acoustical waves of said annular radiation pattern in phase with the acoustical waves of said annular radiation pattern radiating to the other side of the plane of said annular radiation pattern whereby to produce said umbrella shaped toroidal radiation pattern.
 21. The assembly of claim 19 including shield structure defining a cavity in which said assembly is recessed, said shield structure defining cavity side walls spaced from the source of said pattern a distance equal to m lambda , where m is a whole number.
 22. The meThod of claim 20 wherein the acoustical waves of said annular radiation pattern radiating to one side of the selected plane of said annular radiation pattern are reflected from a plane reflecting surface in phase with the directly radiated waves radiating to the other side of the plane of said annular radiation pattern to reinforce same.
 23. The invention defined in claim 20, wherein said annular shaped radiation pattern is generated by combining a plurality of similar spherical radiation patterns of acoustical energy at said selected wavelength.
 24. The invention defined in claim 23, wherein the acoustical waves of said annular radiation pattern radiating to one side of the selected plane of said annular radiation pattern are reflected in phase with the acoustical waves of said annular radiation pattern radiating to the other side of the plane of said annular radiation pattern by a plane reflecting surface positioned parallel with the plane of said annular radiation pattern and positioned a distance therefrom measured along a normal to said reflecting surface equal approximately to a whole number of wavelengths at said selected wavelength multiplied by the sine of theta , where theta is defined as the angle between the plane of said reflecting surface and the vector of the outwardly radiating energy to be reinforced.
 25. The invention defined in claim 24, wherein the angle theta is 45*.
 26. The invention defined in claim 22, wherein the plane of said annular radiation pattern and said plane reflecting surface are spaced apart a distance measured along a normal to said reflecting surface equal approximately to a whole number of wavelengths at said selected wavelength multiplied by the sine of 45*.
 27. The assembly of claim 21 wherein the cavity side walls are parallel to the selected axis.
 28. The method of claim 22 including reinforcing the directly radiating waves of said annular radiation pattern radiating to the other side of the plane of said annular radiation pattern by also reflecting acoustical waves of said annular radiation pattern by a plane reflecting surface positioned parallel to the selected axis.
 29. A piezoelectric transducer assembly for generating an umbrella shaped toroidal radiation pattern of acoustical energy around a selected axis, comprising: means for generating an annular shaped outwardly radiating pattern of acoustical energy at a selected wavelength, said generating means including a piezoelectric transducer element and being positioned to generate said annular shaped radiation pattern concentric with said selected axis, and, means defining a plane reflecting surface perpendicular to said selected axis positioned with respect to said generating means for reflecting a selected portion of said annular shaped radiation pattern to form therefrom said umbrella shaped toroidal radiation pattern around said selected axis. 